Time expressions show whether an action happened in the past or the present. For example, when you use the phrase ‘two years ago’, you are obviously referring to the past. On the other hand, if you say ‘next week’, the reference is to the future. You have to use a past tense with a time expression referring to the past. In the same way, you have to use a future tense with a time expression referring to the future.
I met him last week. (Past tense)
I will meet you next week. (Future tense)
Note that sometimes we use present tenses to refer to the future.
Time expressions are not exactly tricky, but sometimes they do cause confusion.
Here is an overview of commonly confused time words.
In and during
These two time prepositions are often interchangeable.
Both in and during can be used to say that something happens inside a particular period of time.
I was on holiday during May. OR I was on holiday in May.
In can also show position. During cannot be used with this meaning.
I saw him in his office. (NOT I saw him during his office.)
During is preferred when we are stressing that we are talking about the whole of a period.
He was in the hospital during the whole of August. (NOT He was in the hospital in the whole of August.)
Before and ago
Before and ago are often confused.
Ago goes after an expression of time.
I met him two days ago.
She died five years ago.
Before can be used to introduce an adverb clause of time.
I had finished before he arrived.
I will call you before I come.
Note that in clauses introduced by before, we usually use a present simple or past simple tense.
We should discuss this before we begin the meeting.
While and during
While is a conjunction. It should be followed by a clause. During is a preposition. It should be followed by a noun or pronoun.
We discussed the matter while we were waiting for the train. (While is followed by a clause.)
We discussed the matter during our meeting. (During is followed by a noun.)